(1) Field of the Invention
This invention relates to apparatuses and methods for determining liquid viscosity and/or surface tension. In particular it relates to apparatuses and methods for determining qualitatively and quantitatively oil quality and degradation during frying.
(2) Description of Related Art
In the food industry and catering sector deep-frying involves using the same oil (oil is used herein to refer to edible oils, fats, shortenings and mixtures thereof) for several frying batches. This process results in serious chemical degradation of the oil involving oxidation, di- and oligo-polymer generation, hydrolysis and change in its physical properties. Among these changes, probably the most noticeable is that of viscosity which can even double during a prolonged frying process. The increase in viscosity reflects the generation of di- and oligo-polymers of triacylglycerols. It has been demonstrated that oil degradation due to frying has harmful effects on the human health. Health concern is becoming even more important if one takes into account the fact that consumption of fried foods out of home continues to increase. To prevent frying in oils unfit for human consumption and protect the consumers' health several countries have adopted recommendations and legal limits. According to those limits an oil or fat used for frying should be discarded and replaced by fresh when the total polymers have reached 12-15% per mass or the polar compounds 23-29% (depending on the country); other countries use viscosity as a criterion. It is, thus, important to detect the above thresholds in a reliable way in order to replace the oil as soon as it becomes necessary to do so.
In the case of food industry, determining the quality of an oil used for frying and complying with the above rejection limits is financially and technically feasible because quality control labs staffed with specialized personnel exist. There, the standard methods for quality determination of frying oils are performed as a part of routine analysis. Even if these methods are applicable in the food industry they are too much time consuming and they cannot be used for taking decisions during the frying process. On the other hand in the catering sector it is impossible to apply the above methods because apart from being time-consuming, they are expensive and require skilled personnel. As a result, in most restaurants, fast foods, canteens, sandwich places etc. (even in many of the most developed countries) no quality control measures are applied and fried foods can be of questionable quality and even could pose health concerns. In the larger catering units or fast-food chains, where HACCP (Hazard Analysis Critical Control Points) system is applied, the practice is to reject the oil after a few days of use. However, due to the multi-parameter aspect of frying where numerous variables come into play and affect oil degradation (oil composition, composition and quantity of foods being fried, time, temperature, contact with oxygen etc.) it is impossible to predict the level of oil degradation of an oil used for frying. As a consequence, it is impossible to establish adequate frying practices that prevent the use of a frying oil beyond its rejection limit while at the same time do not impose unnecessary oil rejection resulting to an undue increase of economic and environmental costs. From the above it is obvious that oil quality assassment is needed in order to decide when frying oil should be replaced by fresh one. The ideal frying oil quality determination method for both the food industry and the catering-restaurants-fast foods sector would be rapid, simple and reliable. In addition, for the catering sector such a method would also need to be safe for the foods being prepared at the same area and require no laboratory skill or laboratory equipment.
Several rapid tests that determine frying oil quality have been patented or published in scientific journals and some of them exist in the market as well. An independent review of those existing in the market has been recently published by Bansal G. et al. [Evaluation of commercially available rapid test kits for the determination of oil quality in deep-frying operations, Food Chemistry, 121(2010) 621-626]. Existing methods can be classified according to the compounds determined and/or according to the compounds reported by the methods results. According to this classification there are methods that determine:
a) free fatty acids via colorimetric reactions [U.S. Pat. No. 8,325,345; U.S. Pat. No. 4,654,309; FASafe™ (MP Blomedicals, USA), 3M™ Low Range Shortening Monitor (3M, USA)]
b) subgroups of oxidation products: carbonyl compounds via colorimetric reaction [Fritest® (Merck, Germany)], oxidized fatty acids via colorimetric reaction [Oxifrit-test® (Merck, Germany)], peroxides via colorimetric reaction (U.S. Pat. No. 5,670,374).
c) polar compounds via absorbance/colorimetric reaction [TPM very-Fry® (Test kit Technologies Inc. USA)], polar compounds via measurement of electrical properties [U.S. Pat. No. 6,469,521; U.S. Pat. No. 7,523,006; U.S. Pat. No. 7,834,646; U.S. Pat. No. 6,459,995; U.S. Pat. No. 6,436,713; U.S. Pat. No. 7,132,079; U.S. Pat. No. 7,390,666; World Patent WO 2012/036964 A2, World Patent WO 2011/022254 A2, World Patent WO 2010/148133 A1, Capsens 5000 (Center for Chemical Information Technology, Switzerland), FOM 310 (ebro Electronics, Germany), Testo 265 (Testo Inc., USA)], polar compounds via a thin layer chromatography principle (U.S. Pat. No. 7,390,666) polar compounds via measurement of viscosity [Viscofrit®, (Laboratorio de Securidad Alimentaria S.L., Barcelona) Spain Patent ES 1043160)].
d) alkaline compounds via colorimetric reaction (U.S. Pat. No. 4,349,353)
e) color changes (South African Patent P/96/73728)
f) combined results and properties: polar and polymer compounds by dielectric constant and optical transmittance (U.S. Pat. No. 5,818,731), polar and polymer compounds by viscosity and density measurement (Fri-check®, World Patent WO 2000/71989 A1), free fatty acid and polar compounds content via optical absorption and fluorescence (U.S. Pat. No. 8,257,976).
In order for the said rapid tests to be suitable for assessing oil quality it is required that the group of compounds determined (or the property measured) represents adequately oil degradation during frying. It has been long established that free fatty acid concentration in an oil is not a reliable criterion for frying oil quality. The same holds for soap and peroxide concentration. In addition, measurement of oil color is completely unreliable since color is affected by numerous variables and not only oil chemical degradation.
A suitable method for rapid determination of oil quality should provide clear and unambiguous results. Results of methods involving colorimetric reactions can be problematic because the result of the method can be affected by the oil color. The oil color changes during frying and it depends on the oil type, food being fried and level of oil degradation. Furthermore, colors such as yellows, light reds, or light greens may be difficult to read. In addition comparison with a color scale can be subjective.
An appropriate method determining oil quality should determine only the property of the oil affected by oil degradation and not a property affected by other compounds or other external variables. Many of the proposed methods determine electrical properties of oil. It is true that chemical reactions occurring in an oil during frying affect its electrical properties. However, methods determining electrical properties have a serious drawback: the electrical properties of oils are also highly affected by the food moisture retained in the oil; they are also affected by the presence of food particles in the oil. Such effects can induce significant deviations of actual oil degradation. In addition, attachment of solid residues or water droplets (e.g. after cleaning) on the sensor can result in an erroneous measurement.
A certain category of oils “virgin oils” produced only with physical processing have a higher concentration in polar compounds compared to the rest. These polar compounds are not the result of oil quality degradation; they are present in the raw material and are not removed during the mild physical processes used for their extraction. Moreover, in some cases compounds having higher polarity than the oil (e.g. polyphenols in virgin and extra virgin olive oil) are indices of higher than lower oil quality. Therefore in such cases the determination of polar compounds would unduly overestimate degradation.
Regarding the safety of proposed methods, many of them cannot be used close to food preparation areas since they use unsafe, corrosive or explosive chemicals (e.g. Oxifritest®, Fritest®). Furthermore in some of the methods, such as some of those measuring electrical properties, where the probe is inserted directly in the fryer and not replaced after each measurement there are concerns about the effective cleaning of the measuring probe in respect to food safety.
Regarding the simplicity of above tests some of these tests require laboratory skill (FASafe™, TPM Veri-fry®), some require laboratory equipment (TPM Veri-fry®) or require scientific background (Viscofrit®). Therefore it is difficult to use these tests in fast-foods, restaurants or catering units which lack either technical or scientific personnel or laboratory equipment.
What is however most significant regarding rapid tests existing in the market is that in most cases they do not provide accurate results as shown in an independent review (Bansal et al., 2010). Some tests highly overestimate (FASafe™) or highly under estimate results (Oxifrit-Test®). In other tests high differences with standard methods have been found (3M™ Low Range Shortening Monitor). Finally, all methods based on the measurement of electrical properties examined by Bansal et al. (2010) (i.e. Capsens 5000, FOM 310, Testo 265) provide non-satisfactory results for all food/oil combinations.
The above review on existing methods shows that although a lot of rapid methods have been proposed to meet the need of rapid assessment of frying oil quality all above methods present serious drawbacks related to the principle of the method, the reliability and accuracy of results, the safety for use in food preparation/processing areas and the simplicity to use by personnel lacking laboratory skill.
The above review shows also that the principle of viscosity changes has been far less exploited up to date for the development of fast methods, although the di- and oligopolymers of triacylglycerols constitute the newly formed compounds with higher concentration in an oil close to rejection limit. In addition, it has been shown that the di- and oligopolymers of triacylglycerols formed during frying correlate perfectly with the increase oil's viscosity [Kalogianni E. et al., Effect of repeated frying on the viscosity, density and dynamic interfacial tension of palm and olive oil, Journal of Food Engineering 105 (2011) 169-179]. Furthermore, good correlations have been found between the total polymer compounds and polymer compounds and viscosity. Another aspect that should not be disregarded is that oil viscosity has been also related to oil uptake.
To the applicants knowledge the first who exploited the measurement of viscosity in order to assess frying oil quality are Kress-Rogers E. et al. [Development and evaluation of a novel sensor for the in situ assessment of frying oil quality, Food Control, July 1990, 163-178] who adapted a viscosity probe from GEC Marconi Ltd. and developed an in-situ probe for deep-fryers. The method measures the dampening and resonance frequency of vibration of two short vibrating steel tubes excited by piezocrystals. The test is safe and according to the authors the test showed good correlation with several parameters regarding oil degradation, however no independent report has been found on this method. A similar ultrasonic rapid method for viscosity determination in liquids is presented in U.S. Pat. No. 4,721,874. However, this method as presented is not especially adapted for frying oil quality assessment.
Another rapid test based on the change in viscosity principle is the Viscosfrit® test (Laboratorio de Securidad Alimentaria S.L. Barcelona, Spain). The test is simple and safe. It measures the time required to empty a standard funnel-like cone filled with the oil in question. The cone is emptied by gravity through a small calibrated hole at the bottom. The time measured is compared with a table of values in order to decide whether the oil should be discarded or not. The drawback of this test is that the user should know whether monounsaturated or polyunsaturated fatty acids prevail in the oil composition. Therefore, users with lack of food chemistry knowledge such as people working in restaurants have difficulty in using the test.
Finally, the third test based on viscosity (and density) measurement is Fricheck® (World Patent WO 2000/71989 A1). In this test the time required for a piston-like body dropped in a tube containing the oil at 50° C. is related to the viscosity and density of the fluid. The test provides combined results on polymer and polar material. Limited independent research exists on this test reporting that results of this test are very tolerant.
Washburn and Lukas [Washburn E. W., The dynamics of capillary flow, Physical Review, 17 (1921) 273-283] were the first to analyze the rate of liquid penetration in a horizontal tube of capillary dimension. Other researchers studied capillary penetration (or wicking) in capillary tubes as well as in porous systems with pores of capillary dimensions. In these works, the effects of gravity, pore size distribution, inertia effects etc. were examined [van Oss C. J. et al., Determination of contact angles and pore sizes of porous media by column and thin layer wicking, Journal of Adhesion Science and Technology, 6 (1992) 413-428; Marmur A. amd Cohen R. D., Characterization of porous media by the kinetics of liquid penetration: The vertical capillaries model. Journal of Colloid and Interface Science, 189(1997), 299-304; Siebold A. et al., Effect of dynamic contact angle on capillary rise phenomena, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 161 (2000) 81-87; Marmur A., Kinetics of penetration into uniform porous media: testing the equivalent-capillary concept, Langmuir, 19 (2003) 5956-5959]. Scientific knowledge on capillary penetration has been used in the past in order to determine contact angles and pore sizes in porous media and powders (van Oss et al., 1992). To the applicants knowledge no one has used in the prior art the measurement of the rate of capillary penetration in order to determine the viscosity and/or surface tension of a liquid and in particular frying oil quality.